1. Field of the Invention
The present invention generally relates to a carburetor and, more particularly, an air-vent system for a carburetor.
2. Description of the Prior Art
Vapour of fuel generated in a float chamber of a carburetor is generally taken out from the float chamber through a conduit means called an `inner vent` and is led to an inlet portion of the carburetor, i.e. an outlet chamber space of an air cleaner to be mixed with the suction air of an engine. However, in view of an inconvenience that the excessively rich fuel-air mixture is generated at the inlet portion of the carburetor if a large amount of fuel vapour is exhausted toward the outlet chamber space of the air cleaner during stoppage of the engine, the float chamber is generally provided with an outer vent which also opens thereto, said outer vent being connected to a fuel vapour adsorptive vessel like a charcoal canister by a conduit thereby providing for the fuel vapour which is principally generated during stoppage of the engine, to be captured by adsorption. The fuel vapour adsorptive vessel is connected to a purge port which opens to a suction air passage in the carburetor at a position downstream of a throttle valve when it is opened, whereby the fuel held by adsorption in said fuel vapour adsorptive vessel is released from an adsorptive like charcoal when vacuum is applied to said vessel from said purge port during operation of the engine, to be mixed in fuel-air mixture supplied to the engine. The vent passage which extends from said outer vent to said fuel vapour adsorptive vessel is to be opened only when the engine is stopped and, advantageously when the engine is making an idling operation, while it must be closed when the engine is operating with a load higher than an idling operation. Therefore, a control valve is provided in a conduit which connects said outer vent to said fuel vapour adsorptive vessel, said control valve being adapted to selectively intercept said conduit. Conventionally, said control valve is constituted as an electromagnetic valve or a mechanically operated valve. In the former type, the valve is generally interconnected with a key switch of the engine so that the valve is closed when the key switch is closed while the valve is opened when the key switch is opened. The mechanically operated valve is generally interconnected with the throttle valve so that the valve is opened when the throttle valve is in its closed position while the valve is closed when the throttle valve is opened. However, the system employing the electromagnetic valve has the drawback that it is expensive and, furthermore, the control valve is closed to shut down the operation of the outer vent system when the engine is idling, although it is at that time that it is required that the outer vent system operate. This is because the fuel vapour generated in the float chamber causes an excessively rich fuel-air mixture if all of the vapour is exhausted through the inner vent system and mixed with a relatively small amount of suction air during the idling operation of the engine. The control valve which is operated by mechanical interrelation with the throttle valve has the drawback that the structure is complicated with poor durability and poor accuracy in control.